1. Field of the Invention
The present invention is related to a super-low temperature plastic powder manufacturing method using gas, and more particularly is a method of rapidly producing thermoplastic powder with powder granules under 10 microns by melting the plastic granules in a melting furnace and blowing, in order to cause separation, a plastic solution by high pressure gas at a plastic solution outlet of the melting furnace. Finer spherical plastic micro-granules are gradually formed and are treated with super-low temperature liquid nitrogen to give the plastic micro-granules a low elongation rate, low impact resistance, low fracture resistance and characteristics of extreme brittleness and fragility in order to be rolling pressed by a low temperature calendar to obtain the plastic powder.
2. Description of the Prior Art
Thermo-plastic powder has a fracture resistance and a vibration damping capability superior to those of thermosetting plastic. When it is used as a matrix to make fiber reinforced thermo-plastics (FRTP), the products are less fragile, simpler to manufacture, cost less, have longer lives, and are less easily damaged as compared to a compound material of general fiber reinforced thermosets (FRTS). By recycling the thermo-plastics, their use is more acceptable. Therefore, the newly developed aeronautic compound materials, golf clubs, bicycle frames and articles for leisure or sporting such as tennis rackets have largely used FRTP products. The continuous-fiber reinforced thermoplastics will eventually be favored for these products.
In manufacturing a workpiece with the continuous-fiber reinforced thermoplastics (FRTP), the material used normally is not mixed from fiber and plastic. In fact, fiber and plastic are mixed in a given mixing ratio, and are made into a prepreg in the form of a sheet or a braided cylinder. Such FRTP prepreg has uniform composition, and is convenient to store and use.
The methods for producing the continuous-fiber reinforced thermoplastics (FRTP) presently include primarily powder prepregging, hot melting, solution dip prepregging and film calendaring. The powder prepregging method can result in better material performance because melted thermoplastic powder has a very large adhesive strength, and high elongation at break, as shown in the following table 1:
TABLE 1 __________________________________________________________________________ nature of thermoplastics under room temperature (25.degree. C.) tensile tensile elastic tensile strength elongation at modulus impact strength density material (ksi) break (%) (msi) (ft-lbf/in) (g/cm.sup.2) __________________________________________________________________________ Nylon 6 6-14 30 0.38 0.6-2.2 1.12 Nylon 66 11-12 60 0.4 0.55-1.0 1.13 Polycarbonate 9.5 110 0.34 2.3 1.20 (PC) PET 8.5 50 0.4 0.7-1.0 1.34 Polypropylene 4.5-6.0 200 0.1 0.4-1.2 0.89 (PP) PPS 12 5.0 0.48 0.63 1.30 PPO 7.80 50 0.38 5.0 1.10 PEEK 14.5 &gt;40 0.45 1.6 1.28-1.32 UHMWPE 7.0 350 0.10 30 0.93 PSU 10.2 75 0.36 1.2 1.24 (Udel P-1700) __________________________________________________________________________ Data from: Engineered Materials Handbook, Vol. 2, ASM International, 1988
Given that elongation at break of Nylon 6 is 30%, Nylon 66 is 60%, PET is 50%; and some material are over 100%, (for example, PC is 110%, and PP is over 200%), infiltration of thermoplastics and wetting of fiber in the thermo-plastics is not effective. Therefore, the hot melting, the solution dip prepregging or the film calendaring method cannot effectively overcome the above stated defect. If the powder prepregging is used, the powder which is very fine can be uniformly distributed into the scattered bundle of fiber (tow), and is consolidated in the heating and pressurizing processes in manufacturing the workpiece. This yields an FRTP product with a good dipping effect, so that fibers therein can provide the advantage of high strength and high elastic modulus, and the quality of the product can be assured.
Although the powder prepregging method produces a better product with better uniformity, such superiority can only be provided when the size of the granules of the plastic powder is small enough (under 10 microns), so that the granules easily penetrate into the tow. Therefore, it is a key technique in the continuous-fiber reinforced thermo-plastic (FRTP) manufacturing process to use thermo-plastic powder with granules having sizes smaller than 10 microns.
The current techniques for producing thermo-plastic powder must use crushing and grinding to grind the coarse granular thermo-plastic into the fine powder, such as by using a hammer mill or a ball mill. When the ball mill is used to execute the final grinding work, it is more capable of producing thermo-plastic powder with granule sizes smaller than 10 microns. However, grinding time is long, up to 24-48 hours. This is not economical, because when in mass production, consumption of the grinding balls is very large. Further, under the action of the stress of shearing forces during grinding, thermo-plastic often creates a re-melting phenomenon causing the fine powder to agglomerate again, thus enlarging the size of the granules. The beneficial effect of grinding is thereby often largely reduced when grinding the coarse granular thermo-plastic with a ball mill.